The present invention relates to a method of manufacturing aluminum alloy strip for use in the production of lithographic printing plates. The invention also relates to an apparatus for manufacturing such aluminum alloy strip. The invention further relates to aluminum alloy strip for use in the production of lithographic printing plates which is obtained by such a method.
Methods of manufacturing aluminum alloy strip for lithographic printing plates by a continuous casting process typically include a casting step which involves melting an aluminum starting material, subjecting the resulting aluminum melt to filtration treatment, feeding the filtered melt via a melt feed nozzle to a pair of cooled rolls, and solidifying and concurrently rolling the aluminum melt by means of the pair of cooled rolls so as to form an aluminum alloy strip; a cold rolling step; an intermediate annealing step; a finish cold-rolling step; and a flatness correcting step to give an aluminum alloy strip having a thickness of from 0.1 to 0.5 mm. Because these operations are simple, compared with conventional methods of manufacturing aluminum alloy plate for lithographic printing plates that include a direct-chill casting step, a scalping step, a heat soaking step, a heating step and a hot rolling step, the yield is excellent with little loss, the continuous casting process is less subject to fluctuations in the different steps, and the initial equipment costs and running costs are low. On the other hand, because black streaks and other defects specific to continuous casting processes tend to arise, cast strip thus obtained is often unfit for use in the production of lithographic printing plates and other materials which must have a high surface quality.
When continuous casting is carried out, a titanium and boron-containing aluminum alloy is added to the aluminum melt. The TiB2 particles that arise from the titanium and boron-containing aluminum alloy which has been added to the aluminum melt and melted act as a grain refiner. TiB2 particles are, individually, lamellar particles having a size of 1 to 2 μm and a thickness of 0.1 to 0.5 μm, but they readily form agglomerates. If agglomerates having a particle size of 100 μm or more (referred to herein as “coarse TiB2 particles”) are incorporated into the cast strip, when the cast strip is subjected to rolling or annealing or both and finished into a sheet, intermittent black streak-like defects sometimes arise on the surface of the sheet. Such defects are referred to as “black streaks.”
For example, the present inventors earlier disclosed, in JP 3549080 B, a method of manufacturing a lithographic printing plate support which includes a step in which an aluminum melt obtained by the addition of a titanium and boron-containing aluminum alloy is filtered using a filtration tank, then is continuous cast and rolled. In this step, the aluminum melt passes successively through a pre-filter chamber within the filtration tank, a filter which blocks the passage both of single particles 10 μm or larger in size composed of compounds of the titanium and boron present in the titanium and boron-containing alloy and of agglomerates having a particle size of 10 μm or more resulting from the agglomeration of a plurality of such single particles, and a post-filter chamber. At the same time, the pre-filter chamber, the filter and the post-filter chamber are heated by a heater. The same patent publication also discloses, as the filter used in the foregoing method, an aggregation of heat-resistant particles having a size of 5 mm or smaller, and a ceramic tube filter obtained by sintering heat-resistant particles having a size of 0.5 to 2.0 mm.
However, it is known that even with the use of such a fine filter medium, when casting is carried out for a long period of time (i.e., when carrying out continuous casting, such as the casting of more than 50 metric tons), black streaks arises.
In this connection, the inventors have also earlier disclosed in JP 11-47892 A, as a way of preventing black streaks, a continuous casting and rolling apparatus which feeds a melt from a melt feed nozzle to a casting and rolling means, where the melt is then cast and rolled to form a cast strip. This apparatus has formed, at the bottom of a launder through which the melt flows to the melt feed nozzle, a recess in which impurities present in the melt are allowed to settle. The recess has a depth which is from two to five times the depth of the launder, and the recess is open for a length in the direction of flow which is from one to ten times the depth of the launder. However, even when such an apparatus is used, during casting for a long period of time (i.e., during continuous casting, such as the casting of more than 50 metric tons), coarse TiB2 particles which have not settled in the recess become incorporated into the cast strip, leading to the undesirable formation of black streaks.
In addition, the inventors have disclosed in JP 11-254093 A, as a way to prevent black streaks by modifying such a recess, a method of manufacturing aluminum strip using a continuous casting and rolling apparatus provided with, at the bottom of the launder for the aluminum melt, a recess that is notched at a front top edge thereof in the direction of flow, and also a method of manufacturing aluminum strip using a continuous casting and rolling apparatus provided with, at the bottom of the launder for the aluminum melt, a recess that is notched at a back top edge thereof in the direction of flow. However, even using this method, when casting is carried out for a long period of time (i.e., during continuous casting, such as the casting of more than 50 metric tons), black streaks cannot be prevented from forming.
The inventors thus further modified the recess and disclosed in JP 2000-24762 A, as a method for preventing black streaks, a continuous casting and rolling apparatus which feeds the melt from a nozzle to a casting and rolling means, and carries out continuous casting and rolling at the casting and rolling means. The apparatus has, in the recess, a stirring means which agitates the melt in the vicinity of the recess, thereby preventing stagnation in the flow of the melt. However, even with the use of such an apparatus, when casting is carried out for an extended period of time (i.e., during continuous casting, such as the casting of more than 50 metric tons), coarse TiB2 particles that have already settled within the recess swirl up again and are carried downstream, leading to the formation of black streaks.